Just go to profile/settings then signature, then fill it in with words or something.

You can also add a signature pic, but then you need GIMP or PhotoShop. Gimp it's free, but don't have so many tools as PhotoShop. PhotoShop costs VERY MUCH, but it's one of the best picture editing programs in the world.

GIMP offcial site is: Gimp.orgPhotoShop's offcial site is: Adobe.com

You can search in YouTube for some toturials. You can always find something.